Heretofore, generally as a material of a high-frequency core, soft ferrite, high-silicon steel, an amorphous metal, a powder core, and the like have mainly been used. The reason why the above-mentioned materials are used is as follows. In case of the soft ferrite, the material itself has a high specific resistance. In case of other metal materials, the material may be formed into a thin plate or a powder so as to reduce an eddy current although the material itself has a low specific resistance. The above-mentioned materials are selectively used depending upon a working frequency or an intended use. Summarizing the reason therefor, the material high in specific resistance, such as the soft ferrite, has a low saturation magnetic flux density while the material high in saturation magnetic flux density, such as the high-silicon steel, has a low specific resistance. Thus, a magnetic material having both of a high saturation magnetic flux density and a high specific resistance is not yet provided.
In the meanwhile, following dramatic progress in reduction in size and improvement in function of various electronic apparatuses in recent years, a coil and a transformer are required to be reduced in size and to have an inductance under a large direct current. In order to satisfy the above-mentioned demand, it is necessary to simultaneously improve a saturation magnetic flux density and a high-frequency loss characteristic of the core. Further, due to copper loss resulting from an electric resistance of a winding coil, heat generation of the coil or the transformer is increased. Therefore, it is also desired to provide a method for suppressing temperature elevation.
In case of the soft ferrite, improvement of the saturation magnetic flux density is considered but, actually, no substantial improvement is made. In case of the high-silicon steel or the amorphous metal, the material itself has a high saturation magnetic flux density. However, in order to adapt to a high-frequency band, the material must be formed into a thinner plate as the frequency band is higher. A multilayer core using such material is lowered in space factor, which may result in decrease in saturation magnetic flux density. Further, in case of the powder core, it may be possible to achieve a high specific resistance by inserting an insulating material between fine powder particles and to achieve a high saturation magnetic flux density by high-density molding. However, there are difficult problems to solve. That is, a method of improving saturation magnetization of a soft magnetic powder used therefor and a method of forming a high-density molded body while maintaining insulation between powder particles are not established at present.
In order to remedy the above-mentioned problems, in particular, the problem that a magnetic material having both of a saturation magnetic flux density and a high specific resistance is difficult to obtain, proposal is made of a powder core and a method of producing the same in which a metallic glass powder is used as a soft magnetic powder, mixed with an insulating material, and formed into a molded body at a temperature not lower than a normal temperature so as to obtain a soft magnetic material having a high permeability with a relatively excellent frequency characteristic (see Japanese Unexamined Patent Application Publication (JP-A) No. 2001-189211, hereinafter referred to as a patent document 1).
Herein, there are various kinds of alloy compositions collectively called a metallic glass. However, alloy compositions used as the soft magnetic material are restricted to Fe-based alloys which are generally classified into a PePCBSiGa alloy composition and a FeSiBM (M being a transition metal) alloy composition. The patent document 1 uses the former, i.e., an alloy having the FePcBSiGa alloy composition and discloses that, by the use of this soft magnetic material, excellent magnetic characteristics capable of achieving a high specific resistance and a high saturation magnetic flux density are obtained. It is noted here that the latter, i.e., the FeSiBM alloy composition is also disclosed (see Japanese Unexamined Patent Application Publications (JP-A) Nos. 2002-194514 and H11-131199, hereinafter referred to as patent documents 2 and 3, respectively). Further, it is also disclosed to use the soft magnetic material for a core (see Japanese Unexamined Patent Application Publication (JP-A) No. H11-74111, hereinafter referred to as a patent document 4).
On the other hand, it is disclosed to form a winding coil and a metal powder into an integral structure with a reduced size so that d.c. superposition characteristics are improved (see Japanese Unexamined Patent Application Publications (JP-A) Nos. H04-286305 and 2002-305108, hereinafter referred to as patent documents 5 and 6, respectively).
In case of the above-mentioned soft magnetic materials suitable as the high-frequency core, for example, in case of the FePCBSiGa alloy composition disclosed in the patent document 1, magnetic characteristics including a high permeability with a relatively excellent frequency characteristic are obtained. In this case, however, it is necessary to use an expensive metal such as Ga. This results in a problem that the material itself is high in cost and, therefore, promotion of industrial application is inhibited. On the other hand, in the FeSiBM alloy composition disclosed in the patent documents 2 and 3 and considered about application to the core in the patent document 4, the material itself is excellent in economical efficiency. However, in these documents, no technique for obtaining a high specific resistance and a high magnetic flux density is shown (this is presumably because a method of forming a molded body suitable for the alloy composition is not found). Thus, at present, it is difficult to use the material for the high-frequency core and an inductance component using the same. The patent documents 5 and 6 disclose reduction in size of the coil. However, because an existing soft magnetic metal material is used, reduction of loss is not sufficient.
This invention has been made in order to solve the above-mentioned problems. It is an object of this invention to provide an inexpensive high-frequency core made of a soft magnetic material having a high saturation magnetic flux density and a high specific resistance and to provide an inductance component using the same.